Nassau County prosecutors are investigating a Catholic priest who is one of former Mayor Rudy Giuliani’s best friends for allegedly molesting a teenage boy years ago.

As a result, Monsignor Alan Placa, 57, was stripped of his ability to act publicly as a priest by Long Island’s bishop.

Nassau DA Denis Dillon’s spokesman Rick Hinshaw said the probe began after a news story earlier this month about Richard Tollner, a 43-year-old Albany man, who with another man accused Placa of molesting them as teens 25 years ago at St. Pius Preparatory Seminary in Uniondale, L.I.

Tollner has said prosecutors contacted him the day the story appeared, and that he heard from them again Thursday. He reportedly said investigators told him Nassau County’s probe of child molestation by priests in the Diocese of Rockville Centre had expanded to include “financial irregularities.”

Former New York City Corporation Counsel Michael Hess, a friend of Placa’s, said: “Alan denies – 102 percent – that anything happened” in the way of sexual contact.

In April, Placa was axed as vice chancellor of the diocese and removed him from a team that investigated priest molestation allegations.